Contrastive explanations in the context of XAI answer the question of why a decision occurred relative to some other decision that could also have happened for a specific input [1]}. The decision that happened is often called fact, while foil refers to another possible decision it is being contrasted with [2]}.
Contrastive explanations are usually structured as Decision X as compared to decision Y occurred because features \(f_1\)  ... \(f_n\)  are present and features \(f_1^\prime \)  ... \(f_n^\prime \)  are absent [3]}.
They, therefore, highlight the required minimally present and absent features to achieve the given decision.
